i have not personally chopped a tank. but a friend of mine has now chopped 2 tanks for me. and i can tell you, it isnt easy. what angus67 did is much easier. and gets the desired effect.
however, sometimes you want a bit more, since you will be raising the tank you might want to get that extra gas by lowering the tunnel. i also like the way it looks.
my tank that was chopped is peanut tank from a 74 sportster, narrowed the tank and lowered the tunnel. this was done on a sportster chop i did last year this time. the same guy also chopped a stock xs650 tank on a project that i am working on with my brother. that proved to be much more difficult than the peanut tank. he narrowed it up a good bit, and i think he lowered the tunnel a bit too.
i have a peanut tank i got for a build i was working on a while back and ive decided to narrow the tank and lower the tunnel. i'll give it a shot, since it was only 100 bucks if i screw up im not out a ton of money.